1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus, and more particularly, to an optical scanning device that scans a target surface with a light beam and an image forming apparatus including the optical scanning device.
2. Description of the Related Art
In recent years, improvements in a print speed (high speed) and a writing density (high density) have been required in an image forming apparatus, such as a laser printer and a digital copier. To achieve the improvements in the print speed and the writing density, in a conventional technology, an optical scanning device is employed which includes a light source having a plurality of light-emitting elements, and a scanning target surface is scanned with a plurality of light beams emitted from the light-emitting elements at one time.
A semiconductor laser has been generally used as a light source in an image forming apparatus. Although an edge-emitting laser was mainly used as the semiconductor laser, a vertical-cavity surface-emitting laser (VCSEL) has been recently used. Although the maximum number of light-emitting elements that can be arrayed in the edge-emitting laser is about four to eight, it is possible to array more than eight light-emitting elements in the VCSEL. Therefore, the VCSEL is expected to be the light source to achieve the high speed and the high density in the image forming apparatus.
If a light output of the light source fluctuates, a density variation occurs in an output image. Therefore, a conventional optical scanning device employing the edge-emitting laser monitors a light beam emitted from the edge-emitting laser in its backward direction, and performs an automatic power control (APC) thereby preventing the fluctuation of the light output. However, the surface-emitting laser does not emit a light beam in its backward direction because of its structure. Therefore, an optical scanning device employing the surface-emitting laser needs to control a light intensity of the light source in a manner different from the APC performed in the conventional technology.
For example, Japanese Patent Application Laid-open No. H9-288244, Japanese Patent Application Laid-open No. 2002-40350, and Japanese Patent Application Laid-open No. H4-321370 disclose a method of controlling a light intensity of a light source when a surface-emitting laser is employed in an optical scanning device. Specifically, a part of a light beam emitted from the surface-emitting laser is separated and guided to a photodetector by using optical elements, such as a beam splitter and a half mirror, and a drive current of the surface-emitting laser is controlled based on an output of the photodetector.
However, if the requirement of the high speed and the high density in the image forming apparatus becomes higher, it would be difficult for image forming apparatuses disclosed in the above Patent Documents to perform the APC with a sufficient accuracy and support the high speed and the high density. Furthermore, a conventional image forming apparatus has a problem that when the number of light-emitting elements included in a light source becomes larger, a time required for the APC is increased.